1. Field of the Invention
This invention relates generally to apparatus for cutting plastic films. In one aspect, the invention relates to apparatus for perforating thermoplastic films in the manufacture of plastic bags.
2. Description of the Prior Art
In the manufacture of bottom sealed plastic bags, it is necessary to process the bag through an apparatus which produces a heat seal and an adjacent perforated line across the tubular film. The heat seal provides the bottom closure for one bag and the perforations permit the adjacent bag to be separated by tearing along the perforations. The perforated web must have sufficient strength to maintain integrity until it is desired to separate the bags.
One common technique for heat sealing and perforating the bags is to index the tubular film as it emerges from the blown film extrusion system through an in-line bag machine, which is provided with a reciprocating serrated blade and sealing element. With a portion of the flattened film positioned in the bag machine, the blade and heat sealing assembly is actuated. The assembly moves downwardly to cause the heating element to engage the film and the blade to penetrate the film. The assembly then moves upwardly to the retracted position permitting the film to be indexed to the next position, wherein the heat sealing and perforation cycle is repeated. A commercially available machine that operates in this manner is the Gloucester Universal Bag Machine, marketed by Gloucester Engineering Co., Inc., located in Gloucester, Mass.
The in-line bag machine, as described above, functions satisfactorily for many types of film such as low density polyethylene (LDPE) film. However, it has been found that machines of this type are not satisfactory for linear low density polyethylene (LLDPE) film. Experience with LLDPE has shown that it is important that the blades maintain their sharpness to provide satisfactory penetration. It is believed that the superior physical properties of LLDPE over LDPE is much more sensitive to blade sharpness. When the blade becomes even slightly dulled, it tends to push the film downwardly into the slot underlying the film and aligned with the cutting blade, resulting in incomplete perforations. Moreover, the downward pushing action increases the film tension in the heat sealing region and adversely affects the quality of the seal attainable.